1. Field of the Invention
The present invention relates generally to a stepping motor. More particularly, an improved control system for a stepping motor is disclosed.
2. Description of the Related Art
In general, there is known a stepping motor which includes a rotatable rotor having a permanent magnet, stators arranged around the rotor, and induction coils for magnetizing the stators to form N pole or S pole. Some designs have been proposed that stop excitation of the induction coils to prevent the stepping motor from being heated when the rotor stops rotating (at the waiting time) with the motor connected to a power source.
In such designs, the magnetic interaction between the rotor and stator often depends on the relative positioning of the rotor and stator. Thus, if the induction coils are not maintained in the energized state, the rotor cannot stably be stopped at the desired position. Accordingly, it is not possible to restrict the stop position of the rotor with high precision when the coils are deenergized. This hinders fine control of the amount of rotation of the rotor of this motor.
To overcome such a shortcoming, many stepping motors always keep exciting the induction coils even at the waiting time. In order to suppress the heating of the motor at the waiting time as much as possible and prevent unnecessary power consumption, some designs supply a current lower than normal to the induction coils during the waiting time.
In this case, however, since the magnetization of the stators is weaken in accordance with the amount of the current supplied to the induction coils for their excitation, the magnetic balance between the rotor and stators differs from that of the case where the rotor is rotating. It is difficult to sufficiently enhance the accuracy of the position at which the rotor should be stopped.